The present invention relates to a novel process for producing stents and the product of that process. More particularly, the present invention relates to a process for electroforming stents.
Expandable hollow sleeves used as intra vascular endoprosthesis, commonly called stents, are utilized as reinforcements for organ parts in a variety of situations. Typically, they are utilized to support coronary arteries after angioplasty, a process which is used to open clogged arteries. In a typical angioplasty operation, a thin flexible tube, called a catheter, is used to insert a tiny balloon, referred to as an angioplasty balloon, along an artery until it is in a desired position. Once the balloon is in position in an artery, it is inflated so as to open and enlarge the artery, after which the balloon is deflated and removed. This procedure is known to efficiently reopen clogged arteries. However, the arteries thus opened have a tendency to shrink to their former size. Stents are often utilized to hold the arteries open by mechanically supporting the inside of the artery.
A stent used for angioplasty and similar procedures typically is a small, lattice-like tubular structure, on the order of 1/16 inch in diameter and 1/2 inch long. The lattice which forms the tube appears like thin metal wires woven together. The tube lattice is a deformable mesh which permits expansion in the stent's radial dimension. In use, the stent is first inserted over an uninflated angioplasty balloon and then both the stent and balloon are inserted by means of a catheter into the patient. The balloon is inflated, expanding the stent and the artery, after which the balloon is deflated and removed. The expanded stent remains expanded to prevent closure of the artery.
Because of their use internal to the body, stents are subject to a number of rigorous requirements. The stent material must be biocompatible, so that it is neither absorbed by the body nor rejected by the body. Body fluids are highly corrosive to many metals. Thus the stent material must be corrosion resistant to blood and other body fluids. Also, the body's immune system attacks foreign objects. To reduce the risk of such attack, the material of the stent must be inert. The stent material also must be mechanically suitable. It must be sufficiently ductile to be deformed into an expanded condition when the balloon is inflated. It must also be sufficiently rigid to maintain its shape when the balloon is deflated and the artery or the like begins to return to its former size. Because these material constraints vary depending upon the particular application, there is a need for stents to be produced from a variety of metals. Also, blood can easily be damaged by passage through rough, irregular structures and form clots which could clog the artery. Thus, stents must have a very smooth and regular internal surface.
There are a variety of existing methods for forming these stents. In one method, stents have been made using a high power laser to machine slots in a stainless steel tube. This entails using a laser to melt away unwanted portions of the tube, forming a stent lattice. However, accurately positioning and machining a tube in this manner is difficult and the process typically requires manual inspection and processing after the laser machining is performed to remove metal fragments, commonly called slag, from the interior bore of the stent. Slag can take the form of sharp projections that can inhibit blood flow and trigger clotting. Chemical or mechanical removal of slag and inspection to insure a smooth, clean inside surface complicates the laser fabrication process and makes quality assurance and quantity production difficult.
In another method, as noted in U.S. Pat. No. 5,421,955, which is incorporated herein by reference, a mask of acid resistant material is coated onto a metal tube after which a pattern is formed in the mask by use of a laser. A stent is then formed by immersing the masked metal tube into an acid or other metal etching fluid, thus etching away the unmasked material. A limitation of this method is that the etching material eats away at portions of the tube alongside the mask-protected material, allowing the etching material to move under the mask a distance approximately equal to the tube wall thickness. As a result, the cross-section of stent elements formed by etching tends to be nonrectangular and have thin sharp edges. Further, such stents tend to have unpredictable variations in lattice pattern as a result of variations in the amount of material etched away under the mask. The sharp edges and unpredictable patterns created in this process can impede blood flow and damage the blood. Moreover, the etching process limits selection of tube materials to those amenable to etching and also limits tube selection to those with wall thickness which will accommodate the etching process.
Because of the variations inherent in existing processes for producing stents, significant amounts of post-production inspection are required to assure that the required quality is achieved. As a result, existing processes generally require substantial amounts of manual labor to produce completed stents, which results in a relatively high production cost.